An object of the present invention is to provide a process for the treatment of tail gas from a Claus unit or a gas containing essentially hydrogen sulphide and sulphur dioxide as sulphur-containing compounds.
It particularly concerns the treatment of Claus plant tail gas, in particular from hydrodesulphurization and catalytic cracking units. It also concerns the final purification treatment of natural gas. The prior art is illustrated in the following patents: European patent EP-A-0 102 894, French patents FR-A-2 411 802, FR-A-2 532 190, FR-A-2 338 738 and more particularly, French patents FR-A-2 336 163 and FR-A-2 358 186.
French patent FR-A-2 336 163 describes a process for the elimination of sulphur from a charge containing hydrocarbons and hydrogen sulphide by reacting the charge with sulphur dioxide produced ex-situ.
French patent FR-A-2 358 186 describes a process for the elimination of sulphur from a charge containing hydrocarbons and hydrogen sulphide by reacting the charge with SO2 and excess H2S, which are separately introduced. The purified gas containing H2S is treated with amines and H2S is recycled to the inlet of the charge into the absorber.
These processes require separate introduction of sulphur dioxide.
Further, the quantities of solid sulphur to be separated from the solvent are far higher (about 1.5 times greater). This process thus requires the sulphur to be recycled to produce the SO2 required for the reaction, thus requiring bigger apparatus.
The Claus process is widely used to recover elemental sulphur from gaseous charges containing hydrogen sulphide (H2S). However, the fumes emitted by Claus type plants, even after a number of catalytic stages, contain non negligible quantities of acid gas. This Claus plant tail gas must therefore be treated to eliminate the majority of toxic compounds in order to comply with anti-pollution regulations. These regulations are becoming ever more strict and thus existing technology must constantly be improved.
As an example, about 95% by weight of the sulphur present can be recovered from a Claus plant; treatment of this Claus plant tail gas (by means of a Clauspol plant, for example) can, for example, recover up to 99.8% by weight of the sulphur by using the following reaction:             2      ⁢              H        2            ⁢      S        +          SO      2        ⇄            3      ⁢      S        +          2      ⁢              H        2            ⁢      O      
using a reaction medium constituted by an organic solvent and a catalyst comprising an alkaline or alkaline-earth salt and an organic acid. The reaction is carried out in a counter-current reactor-contactor and the temperature is controlled by passing the solvent extracted from the lower extremity of the reactor through a heat exchanger via a circulating pump, in order to obtain higher conversion to sulphur while avoiding the formation of solid sulphur. The sulphur is then recovered in liquid form. While the process performance is high, it is limited by different constraints:
the thermodynamic equilibrium of the reaction is such that the reaction never goes to e completion. Some hydrogen sulphide and sulphur dioxide remain in equilibrium with the sulphur and water formed. Typically, the quantity of sulphur present in the unreacted H2S and SO2 and found in the reaction effluent (Clauspol) corresponds to about 0.05% of the total sulphur in the initial charge to the Claus plant. Better conversion could be achieved at a lower operating temperature but this temperature must be kept above the freezing point of sulphur (about 120xc2x0 C.), otherwise the reactor would be blocked by solid sulphur;
the presence of unseparated liquid sulphur in the reactor-contactor, entrained in the solvent and the catalyst which is circulating and which is recycled to the reactor contactor. Not all the droplets of liquid sulphur are separated from the solvent and the presence of liquid sulphur unavoidably entails the presence of gaseous sulphur in the effluent due to the vapour tension of sulphur. As an example, the quantity of non recovered sulphur attributable to its vapour tension is about 0.1% by weight of the sulphur in the initial charge.
The invention aims to overcome the drawbacks of the prior art.
A further object of the invention is to satisfy the strictest standards demanded in the fight against atmospheric pollution by sulphur-containing compounds.
A still further object is to be able to modify existing installations provided with a Claus plant and a unit for treating the effluents from that plant (Clauspol plant) at very low cost.
We have shown that by eliminating substantially all the sulphur vapour in the effluents from a gas treatment plant, for example effluents from Clauspol plants, up to 99.9% of the total sulphur can be recovered and thus the quantity of sulphur emitted into the atmosphere during incineration of the gas can be minimised.
More precisely, the invention concerns a process for the treatment of a non hydrocarbon-containing gas containing sulphur-containing gases, the majority being hydrogen sulphide and sulphur dioxide, in which the gas is brought into contact with an organic solvent containing a catalyst in at least one gas-liquid reactor-contactor at a suitable temperature and a gaseous effluent substantially no longer containing hydrogen sulphide and sulphur dioxide is recovered, characterized in that a single-phase solution of solvent and sulphur is extracted from the reactor-contactor and a portion is cooled in at least one cooling zone to obtain a suspension of sulphur crystals in the solvent, the crystallised sulphur being separated from the solvent in a separation zone, the sulphur-depleted solvent being recovered and recycled at least in part to the reactor-contactor, and the sulphur being recovered.
The term xe2x80x9cnon hydrocarbon-containing gasxe2x80x9d means a charge containing at most 1% by volume of hydrocarbons, preferably at most 500 ppm of hydrocarbons.
Advantageously, the effluent and solvent containing the catalyst are brought into contact as a counter-current.
Cooling the single-phase solution containing the organic solvent significantly reduces the sulphur content and after separation of the crystallised sulphur, the sulphur-depleted organic solvent can be recycled to the reactor-contactor.
In a further feature of the invention, the sulphur separated from the solvent which is recovered in a minimum quantity of solvent can be settled and melted. It can thus be extracted in liquid form. The solvent phase which is recovered can be at least partially recycled upstream of the cooling zone.
In a further advantageous feature of the invention, the remaining portion of the single-phase solution extracted from the reactor-contactor which is not introduced into the cooling zone can be recycled to the reactor-contactor either after mixing with the sulphur-depleted solvent or separately at a point in the reactor-contactor which is below the point for the introduction of the sulphur-depleted solvent. Only about 3% to 50%, advantageously 5% to 20% and preferably 8% to 15% by weight of the single-phase solution needs to be cooled. These conditions mean that a heat exchanger located in the line for recycling the remaining portion of the single-phase solution to the reactor-contactor can be eliminated. As an example, if about 50% of the solution is cooled, this heat exchange is no longer necessary.
Further, by only desaturating a minor portion of the solution of sulphur, the size and thus cost, of the downstream equipment (exchanger, separator) is reduced.
However, the remaining portion of the single-phase solution can be cooled in a further heat exchanger before recycling it to eliminate at least a portion of the heat of reaction liberated in the reactor. This exchanger is generally placed between the outlet for the single-phase solution from the reactor-contactor and the inlet into the reactor-contactor for the sulphur-depleted recycled solvent. In other words, the heat exchanger can partially contribute to cooling either all of the single-phase solution or the remaining portion of the single-phase solution which is not subjected to principal cooling to eliminate sulphur vapour.
In a further feature, the remaining portion of the sulphur-depleted solvent can be recycled upstream of the cooling zone.
In a still further feature of the invention, the size of the crystals in the suspension of sulphur resulting from the cooling step can be increased in at least one crystallisation reservoir interposed between the cooling zone and the separation zone. They can be left there for a suitable period of time before separation. This favours separation and high efficiency cyclones do not need to be used.
The process of the invention and the means for carrying it out can be adapted to a conventional process and apparatus for the treatment of Claus plant tail gases.
Thus the reactor-contactor generally comprises a heat exchanger or equivalent heating means in its lower portion, more precisely in its settling zone. At least a portion of the sulphur separated from the solvent during the separation step can be introduced and liquid sulphur can be extracted from the settling zone of the reactor-contactor. Existing plant can thus be remodelled.
Since the solvent in the reactor-contactor is below the saturation threshold for sulphur, sulphur no longer deposits in the reactor-contactor packing as it remains in solution.
On the other hand, salts from the catalyst, generally alkali metal sulphates, are produced in the reactor-contactor and a suspension thereof circulates in the circuit. Since there is no longer any risk of blockage by sulphur, these salts can be separated using, for example, suitable filters or a column containing a suitable packing, before cooling the single-phase solution of solvent and sulphur.
The reactor-contactor can be operated at a temperature at which the solvent remains liquid, advantageously 50xc2x0 C. to 130xc2x0 C., preferably 80xc2x0 C. to 125xc2x0 C.
The cooling temperature for the single-phase solution extracted from the reactor-contactor can be down to a temperature close to the freezing point of the solvent, advantageously at a temperature in the range 20xc2x0 C. to 100xc2x0 C., selected as a function of the temperature of the reactor-contactor. This temperature is reached after passage through at least one conventional heat exchanger, or after the injection of water.
The solvents which are. generally used are mono- or polyalkylene glycols, mono- or polyalkylene glycol esters or mono- or polyalkylene glycol ethers such as those described in FR-A-2 115 721 (U.S. Pat. No. 3,796,796), FR-A-2 122 674 and FR-A-2 138 371 (U.S. Pat. No. 3,832,454), hereby incorporated by reference. The catalysts are normally those described in these patents, more particularly the alkaline salts of weak organic acids such as benzoic acid and salicylic acid.
The invention also concerns an apparatus for carrying out the process. More precisely, the apparatus comprises at least one gas-liquid reactor-contactor containing at least one packing, at least one gas supply (3) connected to the reactor-contactor, at least one supply (1) for a solvent containing a catalyst connected to the reactor-contactor, means for evacuating purified gas connected to the reactor-contactor, means for extracting a portion of the single-phase solution connected to the reactor-contactor, means (8) for cooling a solution connected to said extraction means, means (10) for separating sulphur from the solvent containing the catalyst connected to the cooling means (8), means (14) for recycling solvent connected to the separation means (10) and to the solvent supply (1), and means (13) for extracting separated sulphur connected to the separation means. Further, a line (18) for recycling the remaining portion of the single-phase solution is connected to the reactor-contactor.